Apparatuses and methods for transmitting and receiving control signal in analog radio-over-fiber (ROF)-based mobile fronthaul

ABSTRACT

An apparatus for transmitting a control signal in a radio-over-fiber (RoF)-based mobile fronthaul includes: a data channel transmitter configured to generate a data signal at a preassigned frequency or wavelength; a control channel transmitter configured to generate a control signal at a designated frequency or wavelength that is shared with other apparatuses; and a combiner configured to combine the data signal with the control signal.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority from Korean Patent Application No.10-2015-0040994, filed on Mar. 24, 2015, in the Korean IntellectualProperty Office, the disclosure of which is incorporated herein byreference in its entirety.

BACKGROUND

1. Field

The following description relates to analog radio-over-frequency(RoF)-based mobile fronthaul, and more particularly, to apparatuses andmethods for transmitting and receiving a control signal.

2. Description of Related Art

Radio-over-fiber (RoF)-based next generation mobile fronthaultechnologies have drawn increasing attention worldwide for their analogsignal transmission features which allow for simple configuration andeconomical construction and operation of networks.

Currently, mobile communication networks use cloud base stationarchitecture that is divided into digital units (DUs) and radio units(RUs), implementing digital optical transmission based on common publicradio interface (CPRI)/open base station architecture initiative(OBSAI). However, if the exiting digital-based optical transmission isapplied in the 4^(th) generation (4G; LTE/LTE-Z) and 5G mobilecommunication networks, traffic needed for optical transmission andreception increases several tens of times due to digitalizationprocesses, and hence the current optical transmission scheme may bepractically difficult to apply.

The next-generation mobile fronthaul technologies based on analog RoFoptical transmission load digital baseband signals for mobilecommunication on any intermediate frequency (IF) carriers, multiplexesthe signals loaded on the IF carriers, and then transmits a multiplexedsignal by using an inexpensive directly modulated light source andphotodetector, thereby simplifying the design and reducing the cost of aDU/RU platform, as well as preventing degradation of service quality dueto latency in digital signal processing. Also, said mobile fronthaultechnologies support various types of network topologies, such aspoint-to-point, star, and ring topologies.

However, since no method for transferring control signals, such asantenna gains and output powers of RUs, which are used to controloptical transmission or RUs, has been defined yet, a new method forimplementing the control signal transfer in an economical and effectiveway is needed.

In the exiting mobile fronthaul that utilizes the currentCPRI/OBSAI-based digital optical transmission scheme, controlinformation is multiplexed in digital frames in time domain and thenused; but in the analog RoF-based mobile fronthaul, if the exitingtransmission scheme is used, it is not possible to transmit controlinformation, and thus a new type of control information transmissionscheme needs to be defined.

SUMMARY

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter.

In one general aspect, there is provided an apparatus for transmitting acontrol signal in an analog radio-over-fiber (RoF)-based mobilefronthaul, the apparatus including: a data channel transmitterconfigured to generate a data signal at a preassigned frequency orwavelength; a control channel transmitter configured to generate acontrol signal at a designated frequency or wavelength that is sharedwith other apparatuses; and a combiner configured to combine the datasignal with the control signal.

In another general aspect, there is provided an apparatus for receivinga control signal in an analog RoF-based mobile fronthaul, the apparatusincluding: a distributor configured to separate received signals intocontrol signals and data signals; a control channel receiver configuredto only demodulate a signal of a designated frequency or wavelength fromamong the control signals separated by the distributor; and a datachannel receiver configured to demodulate the data signals separated bythe distributor.

In yet another general aspect, there is provided an optical transmissionmethod for an analog RoF-based mobile fronthaul, the opticaltransmission method including: generating a data signal at a preassignedfrequency or wavelength; generating a control signal at a designatedfrequency or wavelength that is shared with other apparatuses; andcombining the data signal with the control signal.

In still another general aspect, there is provided an optical receivingmethod for an analog RoF-based mobile fronthaul, the optical receivingmethod including: splitting received signals into control signals anddata signals; demodulating a control signal of a designated frequency orwavelength which is shared with other apparatuses; and demodulating thesplit data signals.

Other features and aspects will be apparent from the following detaileddescription, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are graphs showing examples of a frequency plan used inintermediate frequency-based transmission of control channel accordingto a first exemplary embodiment.

FIG. 2 is a diagram illustrating a mobile fronthaul network structureover which a control channel is transmitted based on intermediatefrequencies according to a first exemplary embodiment.

FIG. 3A is a diagram illustrating a configuration of a transmissionapparatus for IF-based transmission of a control signal according to thefirst exemplary embodiment

FIG. 3B is a diagram illustrating a configuration of a receivingapparatus for intermediate frequency (IF)-based transmission of acontrol signal according to the first exemplary embodiment.

FIG. 4A is a flowchart illustrating an optical transmission method forIF-based transmission of a control signal according to the firstexemplary embodiment.

FIG. 4B is a flowchart illustrating an optical receiving method forIF-based transmission of a control signal according to the firstexemplary embodiment.

FIG. 5 is a diagram illustrating an example of a frequency plan used innon-return-to-zero on-off keying (NRZ-OOK)-based transmission of controlchannel according to a second exemplary embodiment.

FIG. 6 is a diagram illustrating a mobile fronthaul network structureover which a control channel is transmitted based on NRZ-OOK accordingto a second exemplary embodiment.

FIG. 7A is a diagram illustrating a configuration of a transmissionapparatus for NRZ-OOK-based transmission of a control signal accordingto a secondary embodiment.

FIG. 7B is a diagram illustrating a configuration of a receivingapparatus for NRZ-OOK-based transmission of a control signal accordingto the second exemplary embodiment.

FIG. 8A is a flowchart illustrating an optical transmission method forNRZ-OOK-based transmission of a control signal according to the secondexemplary embodiment.

FIG. 8B is a flowchart illustrating an optical receiving method forNRZ-OOK-based transmission of a control signal.

FIG. 9 is a diagram illustrating an example of a wavelength plan used inWDM-based transmission of a control channel according to a thirdexemplary embodiment.

FIG. 10 is a diagram illustrating a mobile fronthaul network structureover which a control channel is transmitted based on a wavelengthdivision multiplexing (WDM) technology according to a third exemplaryembodiment.

FIG. 11A is a diagram illustrating a transmission apparatus forWDM-based transmission of a control signal according to a thirdexemplary embodiment.

FIG. 11B is a diagram illustrating a configuration of a receivingapparatus for WDM-based transmission of a control signal according to athird exemplary embodiment.

FIG. 12A is a flowchart illustrating an optical transmission method forWDM-based transmission of a control signal according to a thirdexemplary embodiment.

FIG. 12B is a flowchart illustrating an optical receiving method forWDM-based transmission of a control signal according to a thirdexemplary embodiment.

Throughout the drawings and the detailed description, unless otherwisedescribed, the same drawing reference numerals will be understood torefer to the same elements, features, and structures. The relative sizeand depiction of these elements may be exaggerated for clarity,illustration, and convenience.

DETAILED DESCRIPTION

The following description is provided to assist the reader in gaining acomprehensive understanding of the methods, apparatuses, and/or systemsdescribed herein. Accordingly, various changes, modifications, andequivalents of the methods, apparatuses, and/or systems described hereinwill be suggested to those of ordinary skill in the art. Also,descriptions of well-known functions and constructions may be omittedfor increased clarity and conciseness.

The following description relates to an apparatus and method fortransmitting signals that allow a digital unit to control a radio unitin the next-generation analog radio-over-fiber (RoF)-based mobilefronthaul. Here, the control signals may include antenna gains, RFoutput powers, optical link management variables, and so on.

To this end, a transmission apparatus includes a data channeltransmitter, a control channel transmitter, and a combiner, wherein thedata channel transmitter generates a data signal at a preassignedfrequency or wavelength, the control channel transmitter generates acontrol signal at a designated frequency or wavelength which is sharedwith other devices, and the combiner combines the data signal with thecontrol signal.

A receiver apparatus includes a distributor, a control channel receiver,and a data channel receiver, wherein the distributor separates thereceived signal into a control signal and a data signal, the controlchannel receiver demodulates only a signal at a designated frequency ordesignated wavelength, from among control signals from the distributorthat are shared with other devices, and the data channel receiversmodulates the data signal from the distributor.

An optical transmission method includes operations of: generating a datasignal at a preassigned frequency or wavelength; generating a controlsignal at a preassigned frequency or wavelength; and combining the datasignal with the control signal.

An optical receiving method includes operations of: separating areceived signal into a control signal and a data signal; modulating thecontrol signal at a designated frequency or wavelength that is sharedwith other devices; and modulating the data signal.

The aforesaid apparatus and method for transmitting and receivingsignals in the analog RoF-based mobile fronthaul may have threeexemplary embodiments according to communication schemes.

A first exemplary embodiment relates to control channel transmissionbased on intermediate frequency, in which a control signal istransmitted using a designated intermediate frequency. A secondexemplary embodiment relates to digital control channel transmissionbased on non-return-to-zero on-off keying (NRZ-OOK), in which a controlsignal is transmitted via digital baseband. A third exemplary embodimentrelates to control channel transmission based onwavelength-division-multiplexing (WDM), in which a control signal istransmitted using a designated wavelength. Each exemplary embodimentwill be described in detail below.

First Exemplary Embodiment: Scheme for Intermediate Frequency-BasedTransmission of Control Channel

FIGS. 1A and 1B are graphs showing examples of a frequency plan used inintermediate frequency-based transmission of control channel accordingto a first exemplary embodiment. Most intermediate frequencies (IF₁ toIF_(N)) are used to transmit data channels, and control channels areappropriately organized and operated by network service providersaccording to the network topology and operation schemes.

Referring to FIG. 1A, a control channel is assigned in the lowestfrequency region, and referring to FIG. 1B, the control channel isassigned in an intermediate frequency region. Assigning the controlchannel in the lowest frequency region or the highest frequency regionmay simplify the configuration of an electrical filter to extract thecontrol channel, thereby reducing the implementation cost of thetransmission scheme.

FIG. 2 is a diagram illustrating a mobile fronthaul network structureover which a control channel is transmitted based on intermediatefrequencies according to a first exemplary embodiment.

Referring to FIG. 2, the mobile fronthaul network consists of a digitalunit (DU) 10, and radio units RU 20-1, 20-2, and 20-3. The DU 10 islocated in a base station and the RUs 20-1, 20-2, and 20-3 are generallyinstalled indoors and outdoors for general subscriber services. In FIG.2, the DU 10 and the RUs 20-1, 20-2, and 20-3 in the mobile fronthaulnetwork are connected in a star network topology, but the connectionsthereof may be made in various topologies, such as ring network, treenetwork, and point-to-point network topologies.

The DU 10 transmits and receives signals to and from the RUs 20-1, 20-2,and 20-3 using different intermediate frequencies assigned to the RUs20-1, 20-2, and 20-3, for which the DU 10 and the RUs 20-1, 20-2, and20-3 should each use an optical transceiver 11, 21-1, 21-2, and 21-3with control channel demodulation/modulation capability and data channeldemodulation/modulation capability.

Each of the RUs 20-1, 20-2, and 20-3 transmits and receives data to andfrom the DU 10 using an intermediate frequency allocated thereto. Forexample, RU1 20-1 transmits and receives data to and from the DU 10using IF) and IF). RU2 20-2 and RU3 20-3 also have IFs allocated fordata transmission. The number of IFs allocated to each RU 20-1, 20-2,and 20-3 for data transmission and the frequency allocation plan may beadjusted as needed by the network service providers.

Each of the RUs 20-1, 20-2, and 20-3 transmits and receives data via IFsallocated thereto while transmitting and receiving a control signal viathe same IF shared among the RUs 20-1, 20-2, and 20-3. For example, inFIG. 2, each RU receives a control command from the DU, using a controlchannel assigned to a low-frequency band of the RU, and controls theRF-related parameters and parameters necessary for opticaltransmission/reception using the received control command.

A transmission apparatus and a receiving apparatus for transmission of acontrol signal and a data signal between the DU 10 and the RUs 20-1,20-2, and 20-3 will be described with reference to FIGS. 3A and 3B, andan optical transmission method and an optical receiving method will bedescribed with reference to FIGS. 4A and 4B.

FIG. 3A is a diagram illustrating a configuration of a transmissionapparatus for IF-based transmission of a control signal according to thefirst exemplary embodiment.

Referring to FIG. 3A, a transmission apparatus includes a controlchannel transmitter 110, a data channel transmitter 120, an electricalcombiner 130, and a directly modulated light source 140.

The control channel transmitter 110 includes a control signal generator111, a multi-level modulator 112, and an electrical modulator 113.

The control signal generator 111 generates a control signal in a serialbinary format. The multi-level modulator 112 modulates the generatedcontrol signal at multiple levels, and converts the modulated controlsignal into an analog signal. The electrical modulator 113 loads saidmodulated control signal from the multi-level modulator 112 on adesignated IF carrier. Here, the electrical modulator 113 uses an IFcarrier that is predetermined according to the frequency plan for thecontrol signal of FIG. 1A or 1B. Also, the electrical modulator 113 mayuse an oscillator 114 whose voltage can be controlled, so as to selectan optimal IF carrier frequency from a frequency band appropriate to anoperational state of the link.

The data channel transmitter 120 includes a digital baseband signalgenerator 121, a digital modulator 122, and a digital-to-analog (D/A)converter 123.

The digital baseband signal generator 121 generates an in-phase andquadrature-phase (IQ) signal to be transmitted to the RU or the DU. Inthe exemplary embodiment, a plurality of digital baseband signalgenerators 121 may be provided, and multiple parallel signals generatedby the respective digital baseband signal generators 121 are input tothe digital modulator 122.

The digital modulator 122 combines generated single- or multi-channelbaseband signals, and generates a digital IQ signal loaded on a single-or multi-IF carrier that is up-converted to a preassigned IF frequency.The D/A converter 123 with an appropriate sampling rate and bitresolution converts the digital IQ-modulated signal into an analogsignal.

Although not illustrated, the control channel transmitter 110 and thedata channel transmitter 120 each may include an amplifier and anattenuator that each have an appropriate gain.

The electrical combiner 130 receives the control signal output from thecontrol channel transmitter 110 and the data signal output from the datachannel transmitter 120 and combines the two signals in frequencydomain. The directly modulated light source 140 optically modulates thecombined signal output from the electrical combiner 130 and outputs aresultant signal.

FIG. 3B is a diagram illustrating a configuration of a receivingapparatus for IF-based transmission of a control signal according to thefirst exemplary embodiment.

Referring to FIG. 3B, a receiving apparatus for IF-based transmission ofa control signal includes a photodetector 210, an electrical splitter220, a control channel receiver 230, and a data channel receiver 240.

The photodetector 210 electrically converts a received optical signal,and the electrical splitter 220 splits the signal sent from thephotodetector 210 into a control signal and a data signal in frequencydomain.

The control channel receiver 230 includes an electrical demodulator 231,a multi-level demodulator 233 and a control signal modulator 234. Theelectrical demodulator 231 receives signals distributed by theelectrical splitter 220, filters said signals to only pass a controlchannel signal within a predesignated frequency band, and thenfrequency-down-converts the control channel signal to a baseband signal.Also, the electrical demodulator 231 may use an oscillator 232 whosevoltage can be controlled, so as to select an optimal carrier frequencyfor demodulation of a control channel from a frequency band that isappropriate to an operational state of the link. The multi-leveldemodulator 233 receives the signal from the electrical demodulator 231,performs analog-to-digital conversion and multi-level demodulation onthe received signal, and then converts a resultant signal to a controlsignal with a serial binary format. The control signal demodulator 234receives the control signal from the multi-level demodulator 233 anddemodulates serial binary data of said control signal to thereby allowsaid signal to be used in operation of DU or RU.

The data channel receiver 240 includes an analog-to-digital (A/D)converter 241, a digital demodulator 242, and a digital baseband signaldemodulator 243.

The A/D converter 241 converts an analog data signal separated by theelectrical splitter 220 into a digital signal, by using an appropriatesampling rate and bit resolution. In this case, to suppressdeterioration of signal quality due to interference of a controlchannel, high bandpass filters may be provided in front of the A/Dconverter 241. The digital demodulator 242 converts a received digitalIQ signal, which is loaded on a single- or multi-IF carrier, into asingle- or multi-channel baseband IQ signal by frequency-down-convertingthe received digital IQ signal. The digital baseband signal demodulator243 demodulates the signal, which is destined for the RU or DU, to adigital baseband data signal with a binary waveform.

Although not illustrated, for amplification and attenuation, the controlchannel receiver 210 and the data channel receiver 220 may each includean amplifier and an attenuator that each have an appropriate gain.

FIG. 4A is a flowchart illustrating an optical transmission method forIF-based transmission of a control signal according to the firstexemplary embodiment.

Referring to FIG. 4A, a transmission apparatus loads a control signal ona designated intermediate frequency, as depicted in S1010. In detail,the control signal is generated in a serial binary format, multi-levelmodulation and digital-to-analog conversion are performed on thegenerated control signal, and then a resultant control signal is loadedon a designated IF carrier. In this case, for control signaltransmission, a specific IF carrier that is predesignated according to afrequency plan as shown in FIG. 1A or 1B may be used. The transmissionapparatus loads a data signal on an assigned IF, as depicted in S1020.In detail, single- or multi-channel baseband IQ signals to betransmitted to an RU or a DU are combined together to thereby generate adigital IQ signal loaded on a single- or multi-IF carrier that isup-converted to a preassigned IF, and then a digital IQ-modulated signalis converted into an analog signal with an appropriate sampling rate andbit resolution for optical transmission. Here, S1010 and S1020 may beconcurrently performed as shown in FIG. 4A, which is, however, onlyprovided as an exemplary embodiment, and said operations may besequentially performed.

The transmission apparatus electrically combines the control signal andthe data signal in frequency domain, as depicted in S1030, and opticallymodulates the combined signal and outputs a resultant signal, asdepicted in S1040.

FIG. 4B is a flowchart illustrating an optical receiving method forIF-based transmission of a control signal according to the firstexemplary embodiment.

Referring to FIG. 4B, a receiving apparatus electrically converts areceived signal, as depicted in S2010, and then splits the convertedsignal into a control signal and a data signal, as depicted in S2020.

The receiving apparatus filters the spilt signals and demodulates acontrol signal within a predesignated frequency band, as depicted inS2030. In detail, the split control signal is frequency-down-convertedto a baseband signal, the baseband signal goes through analog-to-digitalconversion, and a signal resulting from multi-level modulating thedigital signal is modulated to a control signal with a serial binaryformat, thereby allowing the received signal to be used in operation ofDU or RU. The receiving apparatus demodulates a data signal from thesplit signals, as depicted in S2040. In detail, the split analog datasignal is converted into a digital signal by using an appropriatesampling rate and bit resolution, a digital IQ signal loaded on thesingle- or multi-IF carrier is down-converted in frequency and isconverted into a single- or multi-channel baseband IQ signal, and asignal that is destined for the RU or the DU is demodulated into adigital baseband signal. Here, S2030 and S2040 may be concurrentlyperformed, as shown in FIG. 4B, which is, however, only provided as anexemplary embodiment, and said operations may be sequentially performed.

Second Exemplary Embodiment: Scheme for NRZ-OOK-Based Transmission ofDigital Control Channel

FIG. 5 is a diagram illustrating an example of a frequency plan used inNRZ-OOK-based transmission of control channel according to a secondexemplary embodiment.

Because a bandwidth required for transmitting and receiving controlinformation over a control channel is low, generally less than about 10Mb/s, it is most economical to use a digital baseband signal withoutconversion in transmission of a control channel, as shown in FIG. 5. Ifthe NRZ-OOK coding format, which is most frequently used in digitalsignal transmission, is employed, it is possible to simplify theconfigurations of transmission and receiving apparatuses, which isadvantageous from an implementation point of view. However, controlchannel information loaded on a low frequency band may cause frequencyintermodulation interference in a high frequency band, and henceelectrical power of the control channel needs to be carefullydetermined.

FIG. 6 is a diagram illustrating a mobile fronthaul network structureover which a control channel is transmitted based on NRZ-OOK accordingto a second exemplary embodiment.

Referring to FIG. 6, a mobile fronthaul network consists of a digitalunit (DU) 30 and radio units (RUs) 40-1, 40-2, and 40-3. The DU 30 islocated in a base station which includes a baseband unit (BBU), and theRUs are generally installed indoors and outdoors for general subscriberservices. Here, the DU 30 and the RUs 40-1, 40-2, and 40-3 are connectedin a star network topology, but the connections thereof may be made invarious topologies, such as ring network, tree network, andpoint-to-point network topologies.

The DU 30 transmits and receives signals to and from the RUs 40-1, 40-2,and 40-3 using different intermediate frequencies assigned to the RUs40-1, 40-2, and 40-3, for which the DU 30 and the RUs 40-1, 40-2, and40-3 should each use an optical transceiver 31, 41-1, 41-2, and 41-3with control channel demodulation/modulation capability, as well as datachannel demodulation/modulation capability.

Each of the RUs 40-1, 40-2, and 40-3 transmits and receives data to andfrom the DU 30 using an intermediate frequency allocated thereto. Forexample, as shown in FIG. 6, RU1 40-1 transmits and received data to andfrom the DU 10 using IF₁ and IF₂. RU2 40-2 and RU3 40-3 also have IFsallocated for data transmission. The number of IFs allocated to each RU40-1, 40-2, and 40-3 for data transmission and the frequency allocationplan may be adjusted as needed by the network service providers.

Each of the RUs 40-1, 40-2, and 40-3 transmits and receives data via IFsallocated thereto while they all transmit and receive a control signalthrough a digital baseband.

A transmission apparatus and a receiving apparatus for transmission of acontrol signal and a data signal between the DU 30 and the RUs 40-1,40-2, and 40-3 will be described with reference to FIGS. 7A and 7B, andan optical transmission method and an optical receiving method will bedescribed with reference to FIGS. 8A and 8B.

FIG. 7A is a diagram illustrating a configuration of a transmissionapparatus for NRZ-OOK-based transmission of a control signal accordingto a secondary embodiment.

Referring to FIG. 7A, a transmission apparatus for NRZ-OOK-basedtransmission of a control signal includes a control channel transmitter310, a data channel transmitter 320, an electrical combiner 330, and adirectly modulated light source 340.

The control channel transmitter 310 includes a control signal generator311 to generate a control signal as a digital baseband signal with aserial binary format.

The data channel transmitter 320 includes a digital baseband signalgenerator 321, a digital modulator 322, and a digital-to-analog (D/A)converter 323.

The digital baseband signal generator 321 generates an IQ signal to betransmitted to the RUs or the DU. In one exemplary embodiment, aplurality of digital baseband signal generators 321 may be provided, andmultiple parallel signals generated by the respective digital basebandsignal generators 321 are input to the digital modulator 322. Thedigital modulator 322 combines generated single- or multi-channelbaseband IQ signals to thereby generate a digital IQ signal loaded on asingle- or multi-IF carrier that is up-converted to a preassigned IFfrequency. The D/A converter 323 converts the digital IQ-modulatedsignal into an analog signal by using an appropriate sampling rate andbit resolution for optical transmission.

Although not illustrated, the control channel transmitter 310 and thedata channel transmitter 320 each may include an amplifier and anattenuator that each have an appropriate gain.

The electrical combiner 330 receives the control signal output from thecontrol channel transmitter 310 and the data signal output from the datachannel transmitter 320 and combines the two signals in frequencydomain.

The directly modulated light source 340 optically modulates the combinedsignal output from the electrical combiner 330 and outputs a resultantsignal.

FIG. 7B is a diagram illustrating a configuration of a receivingapparatus for NRZ-OOK-based transmission of a control signal accordingto the second exemplary embodiment.

Referring to FIG. 7B, a receiving apparatus for NRZ-OOK-basedtransmission of a control signal includes a photodetector 410, anelectrical splitter 420, a control channel receiver 430, and a datachannel receiver 440.

The photodetector 410 electrically converts a received optical signalinto an electrical signal, and the electrical splitter 420 splits thesignal output from the photodetector 410 into a control signal and adata signal in frequency domain.

The control channel receiver 430 includes a low bandpass filter 431 anda control signal demodulator 432. The low bandpass filter 431 filtersreceived signals to pass a control channel signal within a designatedbaseband. The control signal demodulator 431 demodulates serial binarydata to thereby allow the received control signal to be used inoperation of DU or RU.

The data channel receiver 440 includes an analog-to-digital (A/D)converter 441, a digital demodulator 442, and a digital baseband signaldemodulator 443.

The A/D converter 441 converts an analog data signal separated by theelectrical splitter 420 into a digital signal by using an appropriatesampling rate and bit resolution. In this case, to suppressdeterioration of signal quality due to interference of a controlchannel, high bandpass filters may be provided in front of the A/Dconverter 441. The digital demodulator 442 converts a received digitalIQ signal, which is loaded on a single- or multi-IF carrier, into asingle- or multi-channel baseband IQ signal by frequency-down-convertingthe received digital IQ signal. The digital baseband signal demodulator443 demodulates the signal, which is destined for the RU or DU, to adigital baseband data signal with a binary waveform.

Although not illustrated, for amplification and attenuation, the controlchannel receiver 410 and the data channel receiver 420 may each includean amplifier and an attenuator that each have an appropriate gain.

FIG. 8A is a flowchart illustrating an optical transmission method forNRZ-OOK-based transmission of a control signal according to the secondexemplary embodiment.

Referring to FIG. 8A, a transmission apparatus generates a controlsignal which is a designated baseband signal, as depicted in S3010. Thetransmission apparatus loads a data signal on an assigned IF, asdepicted in S3020. In detail, single- or multi-channel baseband IQsignals destined for an RU or a DU are combined together to therebygenerate a digital IQ signal loaded on a single- or multi-IF carrierthat is up-converted to a preassigned IF, and then a digitalIQ-modulated signal is converted into an analog signal with anappropriate sampling rate and bit resolution for optical transmission.Here, S3010 and S3020 may be concurrently performed as shown in FIG. 8A,which is, however, only provided as an exemplary embodiment, and saidoperations may be sequentially performed.

The transmission apparatus electrically combines the control signal andthe data signal in frequency domain, as depicted in S3030, opticallymodulates the combined signal and outputs a resultant signal, asdepicted in S3040.

FIG. 8B is a flowchart illustrating an optical receiving method forNRZ-OOK-based transmission of a control signal.

Referring to FIG. 8B, a receiving apparatus electrically converts areceived signal, as depicted in S4010, and then splits the convertedsignal into a control signal and a data signal, as depicted in S4020.

The receiving apparatus filters the split signals to select a controlsignal within a baseband and demodulates the control signal, as depictedin S4030. That is, serial binary data is demodulated into the controlsignal in a baseband, such that the received control signal can be usedin operation of DU or RU. The receiving apparatus demodulates a datasignal from the split signals, as depicted in S4040. In detail, thesplit analog data signal is converted into a digital signal at anappropriate sampling rate and bit resolution, a digital IQ signal loadedon the single- or multi-IF carrier is down-converted in frequency and isconverted into a single- or multi-channel baseband IQ signal, and asignal that is destined for the RU or the DU is demodulated into adigital baseband signal. Here, S4030 and S4040 may be concurrentlyperformed, as shown in FIG. 4B, which is, however, only provided as anexemplary embodiment, and said operations may be sequentially performed.

Third Exemplary Embodiment: Scheme for WDM-Based Transmission of aControl Channel

FIG. 9 is a diagram illustrating an example of a wavelength plan used inWDM-based transmission of a control channel according to a thirdexemplary embodiment. Because generally a bandwidth required fortransmitting and receiving control information over a control channel islow, less than about 10 Mb/s, it is most economical to use a digitalbaseband signal without conversion in transmission of a control channel.However, wide bandwidth may also be used when needed. If the NRZ-OOKcoding format, which is most frequently used in digital signaltransmission, is employed, it is possible to simplify the configurationsof transmission and receiving apparatuses, which is advantageous from animplementation point of view.

FIG. 10 is a diagram illustrating a mobile fronthaul network structureover which a control channel is transmitted based on a WDM technologyaccording to a third exemplary embodiment.

Referring to FIG. 10, a mobile fronthaul network consists of a digitalunit (DU) 50 and radio units (RUs) 60-1, 60-2, and 60-3. The DU 50 islocated in a base station which includes a baseband unit (BBU), and theRUs are generally installed indoors and outdoors for general subscriberservices. Here, the DU 50 and the RUs 60-1, 60-2, and 60-3 are connectedin a star network topology, but the connections thereof may be made invarious topologies, such as ring network, tree network, andpoint-to-point network topologies.

The DU 50 transmits and receives signals to and from the RUs 60-1, 60-2,and 60-3 using different wavelengths assigned to the RUs 40-1, 40-2, and40-3, for which the DU 50 and the RUs 60-1, 60-2, and 60-3 should eachuse an optical transceiver 51, 61-1, 61-2, and 61-3 with control channeldemodulation/modulation capability, as well as data channeldemodulation/modulation capability.

Each of the RUs 60-1, 60-2, and 60-3 transmits and receives data to andfrom the DU 50 using the wavelengths preassigned thereto. For example,RU1 60-1 transmits and receives data to and from the DU 50 usingwavelengths λ₁ and λ₂. RU2 60-2 and RU3 60-3 also have wavelengthsallocated for data transmission. The number of wavelengths allocated toeach of the RUs 60-1, 60-2, and 60-3 for data transmission and thewavelength allocation plan may be adjusted as needed by the networkservice providers.

The RUs 60-1, 60-2, and 60-3 transmit and receive data via wavelengthsallocated thereto, whereas they use the same designated wavelength λ_(C)to transmit and receive a control signal.

A transmission apparatus and a receiving apparatus for transmission of acontrol signal and a data signal between the DU 50 and the RUs 60-1,60-2, and 60-3 will be described with reference to FIGS. 11A and 11B,and an optical transmission method and an optical receiving method willbe described with reference to FIGS. 12A and 12B.

FIG. 11A is a diagram illustrating a transmission apparatus forWDM-based transmission of a control signal according to a thirdexemplary embodiment.

Referring to FIG. 11A, a transmission apparatus includes a controlchannel transmitter 510, a data channel transmitter 520, and awavelength-division multiplexer 530.

The control channel transmitter 510 includes a control signal generator511 and a directly modulated light source 512. The control signalgenerator 511 generates a control signal in a serial binary format. Thedirectly demodulated light source 512 converts a signal output from thecontrol signal generator 511 into an optical signal at a predeterminedwavelength. In this case, the directly modulated light source 512 mayuse a wavelength predetermined according to the wavelength plan for acontrol signal, as shown in FIG. 9.

The data channel transmitter 520 includes a digital baseband signalgenerator 521, a digital demodulator 522, a D/A converter 523, and adirectly demodulated light source 524.

The digital baseband signal generator 521 generates an IQ signal to betransmitted to the RUs or the DU. In one exemplary embodiment, aplurality of digital baseband signal generators 521 may be provided, andmultiple parallel signals generated by the respective digital basebandsignal generators 521 are input to the digital modulator 522. Thedigital modulator 522 combines generated single- or multi-channelbaseband IQ signals to thereby generate a digital IQ signal loaded on asingle- or multi-IF carrier that is up-converted to a preassigned IFfrequency. The D/A converter 523 converts the digital IQ-modulatedsignal into an analog signal by using an appropriate sampling rate andbit resolution for optical transmission. The directly modulated lightsource 524 optically modulates a signal output from the D/A converter523 and outputs a resultant signal.

Although not illustrated, the control channel transmitter 510 and thedata channel transmitter 520 each may include an amplifier and anattenuator that each have an appropriate gain.

The wavelength-division multiplexer 530 receives the control signaloutput from the control channel transmitter 510 and the data signaloutput from the data channel transmitter 520 and combines the twosignals in wavelength domain. In this case, any type of optical combinermay be used as the wavelength-division multiplexer 530.

FIG. 11B is a diagram illustrating a configuration of a receivingapparatus for WDM-based transmission of a control signal according to athird exemplary embodiment.

Referring to FIG. 11B, a receiving apparatus includes awavelength-division demultiplexer 610, a control channel receiver 620,and a data channel receiver 630.

The wavelength-division demultiplexer 610 splits a received signal intoa control signal and a data signal in wavelength domain.

The control channel receiver 620 includes a photodetector 621 and acontrol signal demodulator 622. The photodetector 621 electricallyconverts the control signal output from the wavelength-divisiondemultiplexer 610. The control signal demodulator 622 demodulates theelectrically converted serial binary data to thereby allow the receivedcontrol signal to be used in operation of DU or RU.

The data channel receiver 630 includes a photodetector 631, an A/Dconverter 632, a digital demodulator 633, and a digital baseband signaldemodulator 634.

The photodetector 631 electrically converts the data signal output fromthe wavelength-division demultiplexer 610. The A/D converter 632converts an analog data signal from the photodetector 631 into a digitalsignal by using an appropriate sampling rate and bit resolution. In thiscase, to suppress deterioration of signal quality due to interference ofa control channel, high bandpass filters may be provided in front of theA/D converter 632. The digital demodulator 633 converts a receiveddigital IQ signal, which is loaded on a single- or multi-IF carrier,into a single- or multi-channel baseband IQ signal byfrequency-down-converting the received digital IQ signal. The digitalbaseband signal demodulator 634 demodulates the signal, which isdestined for the RU or DU, into a digital baseband data signal with abinary waveform.

Although not illustrated, the control channel receiver 620 and the datachannel receiver 630 each may include an amplifier and an attenuatorthat each have an appropriate gain.

FIG. 12A is a flowchart illustrating an optical transmission method forWDM-based transmission of a control signal according to a thirdexemplary embodiment.

Referring to FIG. 12A, a transmission apparatus generates a controlsignal which is a designated baseband signal, as depicted in S5010.Then, the transmission apparatus optically modulates the control signalat a designated wavelength, as depicted in S5020.

The transmission apparatus loads a data signal on preassignedintermediate frequencies, as depicted in S5030. In detail, single- ormulti-channel baseband IQ signals destined for an RU or a DU arecombined together to thereby generate a digital IQ signal loaded on asingle- or multi-IF carrier that is up-converted to a preassigned IF,and then a digital IQ-modulated signal is converted into an analogsignal with an appropriate sampling rate and bit resolution for opticaltransmission. The transmission apparatus optically modulates the datasignal, as depicted in S5040. Here, operations S5010 and S5020 andoperations S5030 and S5040 may be concurrently performed as shown inFIG. 12A, which is, however, only provided as an exemplary embodiment,and said operations may be sequentially performed.

Then, the transmission apparatus combines the control signal and thedata signal optically or in wavelength domain, and outputs a resultantsignal, as depicted in S5050.

FIG. 12B is a flowchart illustrating an optical receiving method forWDM-based transmission of a control signal according to a thirdexemplary embodiment.

Referring to FIG. 12B, a receiving apparatus splits a received signalinto a control signal and a data signal in wavelength domain, asdepicted in S6010.

The receiving apparatus electrically converts the control signal, asdepicted in S6020, and filters the converted control signal to select acontrol signal within a baseband and demodulate the selected controlsignal, as depicted in S6030. That is, serial binary data is demodulatedinto the control signal in a baseband, so that the received controlsignal can be used in operation of DU or RU.

The receiving apparatus electrically converts the split data signal, asdepicted in S6040, and demodulates the converted data signal, asdepicted in S6050. Specifically, the split analog data signal isconverted into a digital signal by using an appropriate sampling rateand bit resolution, a digital IQ signal loaded on the single- ormulti-IF carrier is down-converted in frequency and is converted into asingle- or multi-channel baseband IQ signal, and a signal that isdestined for the RU or the DU is demodulated into a digital basebandsignal. Here, operations S6020 and S6030 and operations S6040 and 6050may be concurrently performed, as shown in FIG. 4B, which is, however,only provided as an exemplary embodiment, and said operations may besequentially performed.

According to the exemplary embodiments as described above, a method andapparatus for transmitting various signals (e.g., antenna gains, RFoutput powers, and optical link management variables) for a digital unitto control radio units in a next-generation mobile fronthaul networkwhich utilizes an analog RoF technique to process a large volume of datatraffic are provided, thereby enabling communication service providersto reduce their operational costs for mobile communication basestations.

The current embodiments can be implemented as computer readable codes ina computer readable record medium. Codes and code segments constitutingthe computer program can be easily inferred by a skilled computerprogrammer in the art. The computer readable record medium includes alltypes of record media in which computer readable data are stored.Examples of the computer readable record medium include a ROM, a RAM, aCD-ROM, a magnetic tape, a floppy disk, and an optical data storage.Further, the record medium may be implemented in the form of a carrierwave such as Internet transmission. In addition, the computer readablerecord medium may be distributed to computer systems over a network, inwhich computer readable codes may be stored and executed in adistributed manner.

A number of examples have been described above. Nevertheless, it will beunderstood that various modifications may be made. For example, suitableresults may be achieved if the described techniques are performed in adifferent order and/or if components in a described system,architecture, device, or circuit are combined in a different mannerand/or replaced or supplemented by other components or theirequivalents. Accordingly, other implementations are within the scope ofthe following claims.

What is claimed is:
 1. An apparatus for transmitting a control signal inan analog radio-over-fiber (RoF)-based mobile fronthaul, the apparatuscomprising: a data channel transmitter configured to generate a datasignal at a preassigned frequency or wavelength; a control channeltransmitter configured to generate a control signal at a designatedfrequency or wavelength that is shared with other apparatuses; acombiner configured to combine the data signal with the control signal;and a directly demodulated light source to optically modulate a signaloutput from the combiner, wherein the combiner receives signals from thedata channel transmitter and the control channel transmitter, combinesthe received signals in frequency domain, and outputs a resultant signalto the directly modulated light source, wherein the control channeltransmitter comprises a control signal generator configured to generatethe control signal in a serial binary format, a multi-level modulatorconfigured to convert the control signal with a serial binary formatinto a multi-level modulated signal, and an electrical modulatorconfigured to load the multi-level modulated signal on a designatedintermediate frequency.
 2. The apparatus of claim 1, wherein the controlsignal generator configured to generate the control signal as a digitalbaseband signal with the serial binary format.
 3. The apparatus of claim1, wherein the data channel transmitter comprises a digital basebandsignal generator configured to generate a data signal as a single ormulti-channel baseband in-phase and quadrature-phase (IQ) signal to betransmitted to another apparatus, a digital modulator configured tocombine single or multi-channel baseband IQ signals to thereby generatea digital IQ signal loaded on a single- or multi-IF carrier that isup-converted to a preassigned IF frequency, and a digital-to-analog(D/A) converter configured to convert the digital IQ signal into ananalog signal.
 4. The apparatus of claim 1, wherein the combinerreceives signals output from the data channel transmitter and thecontrol channel transmitter, combines the received signals in wavelengthdomain and outputs a resultant signal, and the control channeltransmitter comprises a control signal generator configured to generatethe control signal as a digital baseband signal with a serial binaryformat, and a directly modulated light source configured to convert thesignal output from the control signal generator into an optical signalwith the designated wavelength.
 5. The apparatus of claim 1, wherein:the combiner receives signals output from the data channel transmitterand the control channel transmitter, combines the received signals inwavelength domain, and outputs a resultant signal, and the data channeltransmitter comprises: a digital baseband signal generator configured togenerate a data signal as a single- or multi-channel baseband IQ signalto be transmitted to another apparatus, a digital modulator configuredto combine generated single- or multi-channel baseband IQ signals tothereby generate a digital IQ signal loaded on a single- or multi-IFcarrier that is up-converted to a preassigned IF frequency, a D/Aconverter configured to convert the digital IQ signal into an analogsignal, and a directly modulated light source configured to opticallymodulate the analog signal from the D/A converter and output a resultantsignal.
 6. An optical transmitting and receiving method for an analogRoF-based mobile fronthaul, the optical transmitting and receivingmethod comprising: transmitting a signal; and receiving the signal,wherein the transmitting of the signal comprises generating a datasignal at a preassigned frequency or wavelength; generating a controlsignal in a serial binary format at a designated frequency or wavelengththat is shared with other apparatuses; converting the control signalwith a serial binary format into a multi-level modulated signal; loadingthe multi-level modulated signal on a designated intermediate frequency;combining the data signal with the control signal to generate thesignal; outputting a resultant signal; and optically modulating a signalresulting from the combination of the data signal and the controlsignal, wherein the control signal and the data signal are combined infrequency domain.
 7. The optical transmitting and receiving method ofclaim 6, wherein the transmitting of the signal further comprisesoptically modulating a signal resulting from the combination of the datasignal and the control signal, wherein the control signal and the datasignal are combined in frequency domain, and the control signal isgenerated at a designated intermediate frequency or as a basebandsignal.
 8. The optical transmitting and receiving method of claim 6,wherein the data signal and the control signal are combined inwavelength domain, and the control signal is generated as a designatedfrequency signal.
 9. The optical transmitting and receiving method ofclaim 6, wherein the receiving of the signal comprises splitting thereceived signal into the control signal and the data signal;demodulating the control signal of the designated frequency orwavelength which is shared with other apparatuses; and demodulating thesplit data signal.
 10. The optical transmitting and receiving method ofclaim 9, wherein the receiving of the signal further compriseselectrically converting the received signal before splitting, whereinthe splitting of the received signal comprises splitting theelectrically converted signal into the control signal and the datasignal in frequency domain, and the demodulation of the control signalcomprises demodulating only the control signal of the designatedfrequency or a baseband signal.
 11. The optical transmitting andreceiving method of claim 9, wherein the splitting of the receivedsignal comprises splitting the received signal into the control signaland the data signal in wavelength domain, and the demodulation of thecontrol signal comprises electrically converting the split controlsignal and demodulating the control signal of the designated frequencyfrom the electrically converted signal.